1. Field of the Invention
The present invention relates to a switching power supply (SPS), and more particularly, to control for a switching power supply (SPS) having an automatic burst mode operation.
2. Description of the Related Art
In general, a switching power supply (SPS) is a device for converting a direct current (DC) supply voltage into one or more DC output voltages that have a greater or lesser magnitude than the DC supply voltage. Such SPSs have been widely used with power electronic devices, particularly battery-powered devices, such as portable cellular phones and laptop computers. Typically, these SPSs have a normal operation mode and a standby operation mode. In the normal operation mode, the power electronic devices consume a relatively large amount of power. In the standby operation mode, by contrast, the power electronic devices consume a relatively small amount of power. These power electronic devices may automatically enter the standby operation mode if a user has not used them for a predetermined amount of time and may automatically enter the normal operation mode if the user begins to use them again.
In most electronic devices, power consumption in the standby operation mode is much smaller than that in the normal operation mode. To reduce power consumption in the standby operation mode, the control of input power in the standby operation mode has been gradually tightened. Conventionally, to meet such control requirements, an output voltage of the SPS is reduced or an additional power source is used to reduce power consumption in the standby operation mode. However, such approaches for entering the standby operation mode are not desirable because they require additional components, thereby resulting in high production costs. Moreover, because there is an output voltage below the level needed for functioning of the electronic devices, the extent to which power consumption of the electronic devices can be decreased is limited. Also, when using a conventional SPS in the standby operation mode, a substantial switching loss occurs in the conventional SPS despite the reduction in power consumption due to a reduced output voltage. In addition, the conventional SPS changes a duty cycle of a power switch in order to compensate for changes in power requirements at its output end and operate at a predetermined frequency irrespective of the amount of power supplied. As a result, the power switch within the conventional SPS operating in the standby operation mode performs switch-on/off operations at the same rate as when the SPS is operating in the normal operation mode. In the standby operation mode, such switching operations consume a considerable amount of power, which increases the lower limit of power consumption in the standby operation mode.
In an attempt to solve such problems, a switching power supply (SPS) has been proposed which uses active circuitry to provide the normal operation mode and a low power burst mode that enables a standby operation of an electronic device. U.S. Pat. No. 6,252,783 discloses the operation and configuration of such an SPS. In the normal operation mode, the active circuitry couples an output voltage of the SPS to a conventional switch driver circuit (or a control module circuit). This switch driver circuit changes a duty cycle of an output having a fixed frequency of a switch driver in order to regulate the output voltage of the SPS to a desired level. When the electronic device is operating in the low power burst mode, the active circuitry separates the output voltage of the SPS from the switch driver circuit and applies a periodic signal to the switch driver. This periodic signal causes the switch driver to provide the output having the fixed frequency for certain time intervals. These time intervals are interleaved with time intervals during which the output of the switch driver is inactive—i.e., the switch driver turns the switch off. In addition, when the electronic device is operating in the low power burst mode, the active circuitry applies an input signal to the switch driver. This signal causes the switch driver to repeatedly turn the switch on and off at a fixed frequency—i.e., at a minimum duty cycle. The low power burst mode during which the switch driver provides the output having the fixed frequency is appropriately controlled so that a supply voltage to the switch driver changes between two reference voltages.
Such SPS having the low power burst mode reduces the switching loss by performing and stopping switch-on/off operations for a predetermined amount of time in the standby operation mode. This reduces power consumption. In addition, the SPS can maintain an output voltage in the standby operation mode at a lower level than in the normal operation mode and can control the switch-on/off operations of the switch in the standby operation mode by using the low power burst mode at a predetermined time interval irrespective of the output voltage of the SPS.
However, such SPS having the low power burst mode may have audible noises as the maximum amplitude of current increases. As the maximum amplitude of current increases, so does the importance of the switching loss. The SPS may have conduction loss and core loss as well as switching loss. When a light load is used by the entire system, the switching loss increases relatively. Thus, when using the light load, frequent audible noises are caused and more power is consumed with the increase in the maximum amplitude of current.